1. Field of the Invention
This invention relates to a thermal process for recovering products from oil shale and, more particularly, to a novel process for improving production of lower molecular weight products from oil shale by volatilizing the shale oil and refluxing a portion of the higher molecular weight fractions.
2. The Prior Art
Oil shale is defined as a fine-grained, sedimentary rock having splintery, uneven fractures and including an organic material generally referred to as kerogen. Kerogen is a ruberoid material with a ratio somewhat higher than conventional petroleum. Shale oil is produced from oil shale be destructive distillation of the kerogen, normally by thermal means. Oil from oil shale deposits within the United States alone constitutes a potential energy resource of about 27 trillion barrels (nearly triple the equivalent energy contained in the domestic coal reserves or 130 times the crude oil production resource of the United States). For example, the oil shale lying within the Green River Formation (located in the states of Utah, Colorado and Wyoming) is of sufficient yield and accessibility to be considered recoverable within the realm of present technology and is estimated to be as high as 760 billion barrels. When considered in light of the present economics and the fact that the current technology restricts the recovery of this vast resource only to those relatively shallow, thick veins of high grade oil shale located within the region, this represents a valuable resource. If effective processing of lower grade shale can be realized, the magnitude of this resource may double.
A number of processes have been developed to extract shale oil from shale by retort processes which usually involved heating the raw oil shale and recovering the volatilized products. Thus, the retort processes involve equipment that basically consists of a heat source and a heat exchanger. The heat source is primarily obtained by burning combustible components of the shale oil. These combustible components include: (1) the light gaseous hydrocarbons evolved during the retorting process, (2) the shale oil itself, and (3) the carbon residue left in the inorganic shale matrix after heating and the volatilization of shale oil has been completed. Oil shale retort processes can be classified as either above ground or in situ (underground) processes. While above ground processing appears attractive in terms of efficiency and utilization of available technology, in situ retorting has the obvious advantage of lower mining costs and the elimination of the problem of spent shale disposal.
One in situ retorting process has been tested wherein hot methane was injected into a naturally permeable, leached oil shale formation. This process produced a low pour point oil. However, due to the loss of the injection gas (methane) into the unconfined fracture pattern, this method of recovery proved to be too costly. Super-heated steam is currently being considered as an alternative injection gas to the hot methane. However, the results are not yet available as to the long range economics of the process particularly as to water loss and energy required to produce the steam.
Another process demonstrated on a commercial scale involved the initial mining of a predetermined volume of oil shale from the top section of an underground body of oil shale. Explosives were then used to rubblize the oil shale body to produce a packed bed column of known void fraction and particle size. A combustion zone was then established at the top of the rubblized column. Combustion of residual carbon in the shale was maintained by the continued injection of air, partially diluted with recycled off gas. The necessary retort heat was provided by the combustion front which moved downwardly through the rubblized oil shale bed heating the raw oil shale directly beneath. The shale oil, initially in vapor form, condensed on the raw shale and drained to the bottom where it was removed. Although this process involved substantial mining and, therefore, was more expensive than a true in situ process, the mining costs were relatively less than any above ground processing. Additionally, spent shale disposal was avoided since the processed shale remained underground.
While it has been demonstrated that shale oil can be produced in commercial quantities with several different processes, the primary obstacle in the path of ultimate large scale utilization of shale oil remains in the fact that shale oil is of a different chemical composition than the average petroleum crude oil. In particular, shale oil contains up to 2% nitrogen (the average for petroleum crude being less than 0.9% nitrogen). Nitrogen tends to form oxides of nitrogen when the product is burned with air so that the use of shale oil as a boiler fuel may face difficult pollution constraints. Nitrogen also acts as a catalyst poison in conventional refineries.
Shale oil also contains a larger percent of residual fractions than conventional crudes. Residual fractions in shale oil are of normally low economic value, so that the market value of shale oil is expected to be less than standard crude oil.
While the first problem, that of high nitrogen content, can be solved by utilizing special denitrification techniques, the solution to the problem of high residual fractions in the shale oil presents a problem which is not overcome in any of the existing retort processes.
In view of the foregoing, it would be an advancement in the art to provide an improved process for recovering products from oil shale. It would also be an advancement in the art to provide a process whereby high residual fractions in shale oil are reduced during the retort process. It would also be an advancement in the art to provide a process for recovering shale oil wherein the off gas recovered therefrom is enriched with hydrogen. Such a process is disclosed and claimed herein.